The present invention relates to methods of, and apparatus for identifying personnel by fingerprint minutia and, more particularly, to a novel method and apparatus for utilizing images of different frequency content, obtained with light polarized in first and second substantially orthogonal directions, to enhance fingerprint minutia edge detail.
Personnel identification by recognition of fingerprint detail is highly desirable under certain circumstances. Contactless detection of fingerprint detail by optical imaging is, however, normally quite difficult due to the barely different reflective properties of the ridges and valleys of the finger tissue. The resulting contrast is often only slightly greater than the image noise, creating great difficulty in minutia recognition. Thus, video imaging is not easily accomplished by the so-called "frustrated total internal reflection" method, in which the finger is pressed against a three-sided transparent prism; the finger is illuminated by a light source directed upon a prism side other than that side against which the finger is pressed, and the image is collected from the remaining prism side. Total internal reflection is frustrated when skin tissue contacts the glass, so that a bright image of the print is seen against a dark background. This method suffers from many contact-related problems so that the quality of the resultant image is acceptable only for a narrow range of finger temperature, dryness and cleanliness. A contactless method, and especially one with relatively high edge detail, is highly desirable.